A bandgap voltage reference circuit generates a reference voltage that is substantially temperature-independent over a desired temperature range and is widely used in integrated circuits.
In some techniques, two components contribute to the output voltage of a bandgap voltage reference. One component is the base-emitter voltage (Vbe) of a diode-configured transistor. The second component is proportional to absolute temperature (PTAT) and is used to compensate for the negative temperature coefficient of Vbe. By multiplying the PTAT voltage with an appropriate factor and summing with Vbe, the bandgap voltage reference will have a low sensitivity to temperature variation.
For example, the voltage difference ΔVbe between two p-n junctions (e.g., diodes), operated at different current densities, can be used to generate a proportional to the absolute temperature (PTAT) current in a first resistor. The PTAT current can be used to generate a voltage in a second resistor. This voltage, in turn, is added to the voltage across one of the junctions. As the voltage across a diode operated with a PTAT current is complementary to absolute temperature (CTAT), if the ratio between the first and second resistors is chosen properly, the first order effects of the temperature dependency of the diode and the PTAT current will cancel out.
It is known, however, that even for a bandgap with an optimally chosen reference temperature T0, the output voltage as a function of temperature displays a curvature that causes it to decrease for temperatures higher or lower than T0 (see FIG. 1). The deviation in output voltage indicated by the curvature as the temperature varies is too large for many applications. Thus, it is desirable to incorporate a curvature correction technique so as to provide a bandgap voltage reference that displays even less temperature, sensitivity.